Power cell bypass method and apparatus for multilevel inverter

ABSTRACT

Multilevel inverters, power cells and bypass methods are presented in which a power cell switching circuit is selectively disconnected from the power cell output, and a bypass which is closed to connect first and second cell output terminals to selectively bypass a power stage of a multilevel inverter, with an optional AC input switch to selectively disconnect the AC input from the power cell switching circuit during bypass.

BACKGROUND

Multilevel inverters are sometimes employed in motor drives and other power conversion applications to generate and provide high voltage drive signals to a motor or other load in high power applications. One form of multilevel inverter is a Cascaded H-Bridge (CHB) inverter architecture, which employs multiple series-connected power stages such as H-Bridge inverters for driving each motor winding phase. Each H-Bridge is powered by a separate DC source and is driven by switch signals to generate positive or negative output voltage, with the series combination of multiple H-Bridge stages providing multilevel inverter output capability for driving a load. Device degradation within a particular power stage, however, may inhibit the ability to provide a desired output voltage to a load, particularly since the stages are connected in series with one another. Accordingly, it is desirable to provide the ability to bypass a particular degraded power stage, for example, to continue operation of a multilevel inverter at reduced output capacity and/or to bypass one or more healthy power stages to balance a power converter output to accommodate one or more degraded power stages that have also been bypassed.

SUMMARY

Various aspects of the present disclosure are now summarized to facilitate a basic understanding of the disclosure, wherein this summary is not an extensive overview of the disclosure, and is intended neither to identify certain elements of the disclosure, nor to delineate the scope thereof. Rather, the primary purpose of this summary is to present various concepts of the disclosure in a simplified form prior to the more detailed description that is presented hereinafter. The present disclosure provides bypass apparatus and techniques for CHB and other power stages or power cells of a multilevel power converter, in which the H-Bridge or other switching circuit is electrically disconnected from the power stage output, and a bypass switch is closed across first and second output terminals in order to bypass the stage. Unlike conventional approaches that simply utilize a bypass switch across the output terminals, the novel technique of the present disclosure advantageously avoids or mitigates allowing a motor or other output load to remain electrically connected with a failed cell.

A power conversion system is provided, which includes multiple series-connected power stages, individually including a switching circuit, a pair of output control switches coupled between the switching circuit and the power stage output, as well as a bypass switch connected across the output. The controller selectively bypasses at least one of the power stages by opening the corresponding output control switches, and closing the bypass switch. In certain embodiments, the output control switches are opened before closing the bypass switch. Certain embodiments, moreover, further provide an input switch coupled between an AC input and the switching circuit of the power stage, where the controller bypasses the power stage by opening the output control switches, closing the bypass switch, and opening the input switch.

A power cell is provided in accordance with further aspects of the disclosure, which may be used as a power stage in a multilevel inverter circuit. The power cell includes an AC input, a rectifier, a DC link circuit, and a switching circuit with two or more switching devices coupled between the DC link circuit and an output. First and second output control switches are connected between corresponding switching circuit nodes and output terminals, and a bypass switch is coupled across the output. Certain embodiments further include an input switch coupled between the AC input and the switching circuit.

Methods are disclosed for bypassing a power stage of a multilevel inverter circuit, including electrically disconnecting a switching circuit of the power stage from an output of the power stage, and electrically connecting two output terminals of the power stage to one another to bypass the power stage. In certain embodiments, the method further includes electrically disconnecting the switching circuit from an input.

BRIEF DESCRIPTION OF THE DRAWINGS

The following description and drawings set forth certain illustrative implementations of the disclosure in detail, which are indicative of several exemplary ways in which the various principles of the disclosure may be carried out. The illustrated examples, however, are not exhaustive of the many possible embodiments of the disclosure. Other objects, advantages and novel features of the disclosure will be set forth in the following detailed description when considered in conjunction with the drawings, in which:

FIG. 1 is a schematic diagram illustrating a three-phase 13-level CHB inverter-based motor drive power conversion system with a controller providing switching and bypass control signals to the individual power cells;

FIG. 2 is a schematic diagram illustrating an H-Bridge power cell or power stage in the power converter of FIG. 1 with a three-phase rectifier, a DC link circuit, an inverter, output control switches coupled between the inverter and the load, and a bypass switch for bypassing the power cell;

FIG. 3 is a flow diagram illustrating an exemplary method for bypassing a power cell in a multilevel inverter power conversion system; and

FIG. 4 is a schematic diagram illustrating another H-Bridge power cell embodiment with output control switches, a bypass switch, and a multi-phase input switch for bypassing the power cell.

DETAILED DESCRIPTION

Referring now to the figures, several embodiments or implementations are hereinafter described in conjunction with the drawings, wherein like reference numerals are used to refer to like elements throughout, and wherein the various features are not necessarily drawn to scale.

FIG. 1 illustrates an exemplary multilevel inverter motor drive power conversion system 10 which includes a three-phase multilevel inverter 40 with series-connected power stages 100-1, 100-2, 100-3, 100-4, 100-5, 100-6 for each of three sections associated with the motor phases U, V and W of a motor load 50. Other embodiments are possible in which other forms of load 50 are driven, wherein the present disclosure is not limited to motor drive type power converters. In certain embodiments, the individual power stages 100 include an H-bridge switching circuit or inverter 140 with switching devices (e.g., Q1-Q4 in FIG. 2 below), although any suitable form of switching circuit 140 may be provided in the individual power stages 100 with two or more switches forming a switching circuit for generating a power stage output having one of two or more possible levels according to switching control signals 222 provided by an inverter control component 220 of a power converter controller 200.

The example of FIG. 1 is a multiphase 13-level inverter 40 with six power stages 100 for each of three motor load phases U, V and W (e.g., 100-U1, 100-U2, 100-U3, 100-U4, 100-U5 and 100-U6 for phase U; 100-V1, 100-V2, 100-V3, 100-V4, 100-V5 and 100-V6 for phase V; and stages 100-W1, 100-W2, 100-W3, 100-W4, 100-W5, 100-W6 for phase W). The various aspects of the present disclosure may be implemented in association with single phase or multiphase multilevel inverter type power conversion systems having any integer number “N” power stages 100, where N is greater than one. In addition, although the illustrated embodiments utilize H-Bridge stages 100 cascaded to form multilevel inverters 40 for each phase of the motor drive system 10, other types and forms of power stages 100 can be used, such as a stage 100 with a switching circuit having more or less than four switching devices, wherein the broader aspects of the present disclosure are not limited to H-Bridge power cells shown in the illustrated embodiments. For instance, embodiments are possible, in which the individual cells may include as few as two switching devices or any integer number of switches greater than or equal to two.

As best seen in FIG. 1, the power converter 10 is supplied with multiphase AC input power from a phase shift transformer 30 having a multiphase primary 32 (a delta configuration in the illustrated embodiment) receiving three-phase power from an AC power source 20. The transformer 30 includes 18 three-phase secondaries 34, with six sets of three delta-configured three-phase secondaries 34, with each set being at a different phase relationship. Although the primary 32 and the secondaries 34 are configured as delta windings in the illustrated example, “Y” connected primary windings and/or secondary windings or other winding configurations can alternatively be used. In addition, while the transformer has three-phase primary and secondary windings 32, 34, other single or multiphase implementations can be used. Although the various secondaries 34 in the illustrated embodiments are phase shifted, non-phase shifted embodiments are possible.

Each three-phase secondary 34 in the example of FIG. 1 is coupled to provide AC power to drive a three-phase rectifier 120 of a corresponding power stage 100 of the three-phase multilevel inverter 40. The inverter 40 is a 13-level inverter with six cascaded H-Bridge power stages 100U-1 through 100U-6 having outputs 104U-1 through 104U-6 connected in series with one another (cascaded) between a motor drive neutral point N and a first winding U of a three-phase motor load 50. Similarly, six power stages 100V-1 through 100V-6 provide series connected voltage outputs 104V-1 through 104V-6 between the neutral N and the second winding V, and power stages 100W-1 through 100W-6 provide series connected voltage outputs 104W-1 through 104W-6 between the neutral N and the third winding W of the motor 50. The controller 200 provides control signals 222U to the power stages 100U-1 through 100U-6 associated with the first motor winding U, and also provides control signals 222V to the power stages 100V-1 through 100V-6 and control signals 222W to the power stages 100W-1 through 100W-6. Although the inverter 40 shown in FIG. 1 is a multiphase unit supplying output power to phases U, V and W to drive a three-phase motor 50, the concepts of the present disclosure are also applicable to single phase converters, for example, a three-phase-to-single phase matrix converter receiving a three phase input from the source 20, with a single series-connected group of cells 100 providing power to a single phase motor or other single phase output load. Moreover, other multiphase outputs can be provided having more than three phases.

Referring also to FIG. 2, power cells 100 are provided for use as the power stages of single or multi-phase multilevel inverters 40, with bypass switching devices actuated by a bypass component 210 of the controller 200. The controller 200 and its components 210, 220 can be implemented using any suitable hardware, processor executed software or firmware, or combinations thereof, wherein an exemplary embodiment of the controller 200 includes one or more processing elements such as microprocessors, microcontrollers, DSPs, programmable logic, etc., along with electronic memory, program memory and signal conditioning driver circuitry, with the processing element(s) programmed or otherwise configured to generate signals 222 suitable for operating the switching devices of the power stages 100. In addition, the illustrated controller 200 in certain embodiments implements the bypass control component 210 to generate bypass control signals 212 for selective bypassing of one or more of the power stages 100.

In certain implementations, the bypass control component 210 provides individual signals or values 212 to the individual power cells 100 for direct control over output control switches S1 a and S1 b (signal(s) 212-1), a bypass switch S2 (signal 212-2) and optionally generates input switching control signal(s) 212-3 for operating an optional input switch S3 (FIG. 4 below). In other possible implementations, local switching driver circuitry and/or switching logic can be provided within the power stages 100 to implement the bypass switching operation as described herein based on one or more initiating actions from the bypass control component 210 or from any other controlling element of, or associated with, the power conversion system 10. For instance, a single signal or value can be provided to an individual power cell 100, and a local logic and/or switching control circuit on the cell 100 can initiate the described bypass switching operation in response to receipt of such a signal or value.

FIG. 2 illustrates one possible implementation of an H-Bridge power stage 100. The power stage in FIG. 2 is implemented as a power cell 100 including an AC input 108 with input terminals 108A, 108B and 108C connectable to receive AC input power, in this case three-phase power from an AC source such as a secondary winding 34 of the transformer 30 in FIG. 1. The AC input power is provided from the cell input 108 to a rectifier circuit 120 having onboard rectifier diodes D1-D6 forming a three-phase rectifier 120 which receives three-phase AC power from the corresponding transformer secondary 34. In this example, a passive rectifier 120 is used, but active rectifier circuits or other forms of rectifiers can be used, whether having a single or multi-phase input. The power cell 100 also includes a DC link circuit 130 and a switching circuit (e.g., H-Bridge inverter 140) providing an output voltage V_(OUT) to a power cell output 104 having first and second output terminals 104A and 104B.

In the illustrated embodiment, the rectifier 120 provides DC power across a DC capacitor C connected between DC link terminals 131 and 132 of the DC link circuit 130. The DC link circuit 130, in turn, provides an input to an H-Bridge inverter 140 formed by four switching devices Q1-Q4 configured in an “H” bridge circuit. Although the illustrated power stage 100 operates based on DC power provided by an internal rectifier circuitry 120 driven by an AC input from the corresponding transformer secondary 34, any suitable form of a DC input can be provided to the power stages 100 in accordance with the present disclosure, and the power stages 100 may, but need not, include onboard rectification circuitry 120. In addition, any suitable switching circuit configuration can be used in the switching circuits 140 (e.g., inverter) of individual stages 100 having at least two switching devices Q configured to selectively provide voltage at the stage output 104 of at least two distinct levels. Moreover, any suitable type of switching devices Q may be used in the power stages 100, including without limitation semiconductor-based switches such as insulated gate bipolar transistors (IGBTs), silicon controlled rectifiers (SCRs), gate turn-off thyristors (GTOs), integrated gate commutated thyristors (IGCTs), etc.

The illustrated four-switch H-Bridge implementation (FIG. 2) advantageously allows selective switching control signal generation by the controller 200 to provide at least two distinct voltage levels at the output 104 in a controlled fashion. For instance, a voltage is provided at the output terminals 104A and 104B of a positive DC level substantially equal to the DC bus voltage across the DC link capacitor C (e.g., +VDC) when the switching devices Q1 and Q4 are turned on (conductive) while the other devices Q2 and Q3 are off (nonconductive). Conversely, a negative output is provided when Q2 and Q3 are turned on while Q1 and Q4 are off (e.g., −VDC). Accordingly, the exemplary H-Bridge power stage 100 advantageously allows selection of two different output voltages, and the cascaded configuration of six such stages (e.g., FIG. 1) allows selective switching control signal generation by the inverter control component 220 to implement 13 different voltage levels for application to the corresponding motor phase U, V or W. It is noted that other possible switching circuitry may be used to implement a two, three, or K-level selectable output for individual stages 100, where K is any positive integer greater than 1. Any suitable logic or circuitry in the controller 200 can be used for providing inverter switching control signals 222 to a given power stage 100, where the controller 200 may also include signal level amplification and/or driver circuitry (not shown) to provide suitable drive voltage and/or current levels sufficient to selectively actuate the switching devices Q1-Q4, for instance, such as comparators, carrier wave generators or digital logic and signal drivers.

For bypassing operation, the power cell 100 in FIG. 2 includes a pair of output control switches S1 a and S1 b coupled between the switching circuit 140 and the output 104. In particular, the first output control switch S1 a is coupled between a first switching circuit node 141 and a first output terminal 104A, and the second output control switch S1 b is coupled between a second switching circuit node 142 and the second output terminal 104B. The output control switches S1 are individually operative in a first state to allow current flow between the switching circuit 140 and the output 104, and in a second state to prevent current from flowing between the switching circuit 140 and the output 104 according to one or more output switching control signals 212-1 from the bypass control component 210. Although a single output control switching signal 212-1 is shown in the example of FIG. 2, separate output control signals 212-1 can be used for the individual switches S1A and S1B in other implementations, which may, but need not be switched at the same time. The output control switches S1, moreover, can be any suitable form of single or multiple electrical or electromechanical switching devices, including without limitation semiconductor-based switches, contactors, relays, etc. In addition, the power cell 100 includes a bypass switch S2 connected across the output terminals 104 and operative according to a bypass control signal 212-2 from the controller 210. The bypass switch S2 is operative in a nonconductive state by which the cell output voltage V_(OUT) is controlled by operation of the switching circuit 140, and a conductive state (e.g., closed or conductive) to bypass the output 104 of the switching circuit 140. The bypass switch 102 can be any suitable form of single or multiple electrical or electromechanical switching device.

In operation of the converter 10, the bypass controller 210 selectively bypasses the cell 100 by placing the first and second output control switches S1 a, S1 b in the respective second states via signal(s) 212-1 and by placing the bypass switch S2 in the conductive state using signal 212-2. Opening the output control switches S1 effectively disconnects and isolates the output 104 (and hence the motor load 50 (FIG. 1)) from the inverter switching circuit 140. Furthermore, closure of the bypass switch S2 effectively electrically connects the first and second output terminals 104A and 104B to one another such that other power cells 100 in the converter 10 can continue to drive the motor load 50 even when a given power cell 100 has been bypassed. As seen in the waveform diagram of FIG. 2, moreover, the controller 200 is operative in certain implementations to selectively place the output control switches S1 a, S1 b in the second state (open) at time T1 before closing the bypass switch S2 at time T2. In this manner, the controller 200 effectively isolates the output 104 from the switching circuit 140 before bypassing the output terminals 104A and 104B. Relative timing and sequence of the switching operations is not critical in all embodiments of the present disclosure, and can be implemented in different orders in other implementations. The difference in the switching times (e.g., T2-T1) for the illustrated embodiments and switching sequences, moreover, can be any suitable length of time controlled by the bypass component 210, for instance, based on the value of the DC link capacitance C or other considerations such as operation of potentially degraded devices within the power cell 100 and/or the need to quickly bypass the power cell 100. In certain embodiments, moreover, the controller 210 may selectively adjust the bypass switching control timing according to one or more conditions in the power converter 10.

FIG. 3 illustrates a power stage bypassing method 300 for bypassing a power stage of a multilevel inverter circuit 40, such as the power cell 100 in FIG. 2 or FIG. 4. In certain embodiments, the controller 200 includes at least one processor programmed to perform the process 300 such as by a bypass control component 210 to provide the signals 212 to select ones of the power cells 100, along with other functionality set forth herein (e.g., providing switching control signals 222 via the inverter control component 220) according to computer executable instructions from a non-transitory computer readable medium, such as a computer memory, a memory within a power converter control system (e.g., controller 200), a CD-ROM, floppy disk, flash drive, database, server, computer, etc. which has computer executable instructions for performing the processes and controller functionality described herein. While the exemplary method 300 is depicted and described in the form of a series of acts or events, it will be appreciated that the various methods of the disclosure are not limited by the illustrated ordering of such acts or events except as specifically set forth herein. In this regard, except as specifically provided hereinafter, some acts or events may occur in different order and/or concurrently with other acts or events apart from those illustrated and described herein, and not all illustrated steps may be required to implement a process or method in accordance with the present disclosure. The illustrated methods may be implemented in hardware, processor-executed software, or combinations thereof, in order to provide the power stage bypassing concepts disclosed herein.

The bypass operation can be initiated according to any suitable input signal received by the controller 200 in certain implementations. For instance, the power conversion controller 200 may detect one or more operating conditions of the power converter 10 indicating possible degradation of one or more power stages 100, and may initiate bypassing of one or more selected cells 100 in response. In other possible implementations, the controller 200 may receive a signal or message from an external device (not shown) and initiate bypassing accordingly.

Bypassing operation begins in the process 300 by opening the output control switches (S1 in FIG. 2) between the inverter switching circuit 140 and the power cell output terminals 104A and 104B at 302 to effectively electrically disconnect and isolate the switching circuit 140 from the output 104. In certain embodiments, one or more input switches (e.g., S3 in FIG. 4 below) are opened at 304 to prevent current flow between the AC input (e.g., the secondary windings 34 in FIG. 1 above) and the rectifier circuit 120 (FIG. 2). At 306, a bypass switch (e.g., switch S2 in FIGS. 2 and 4) is closed in order to bypass the power stage output 104. As discussed above, in certain embodiments, the bypass switch S2 is closed at 306 after opening the output control switches S1 at 302. Moreover, if one or more input control switches S3 are used (FIG. 4), such can be opened at 304 in certain embodiments after the output control switches S1 are opened at 302 and before the bypass switch S2 is closed at 306.

Referring also to FIG. 4, another power cell embodiment 100 is illustrated, which is configured generally as described above in connection with FIG. 2. In addition, the embodiment of FIG. 4 includes an input switch S3 coupled between an AC input 108 and the switching circuit 140 of the power stage 100. The input switch S3 in this case is a three phase (three contact) switching device, but separate switches can be used for each input phase, with two contacts or two switches being used for a single phase AC input, three contacts or three switches being used for a three phase input, etc. Furthermore, for embodiments including an on-board rectifier circuit 120 as shown in FIG. 4, the input switch S3 can be located between the input 108 and the rectifier 120. The input switch S3 is operative in a first (e.g., conductive or ON) state to allow current to flow between the AC input 108 and the switching circuit 140 and in a second (nonconductive or OFF) state to prevent current from flowing between the AC input and the switching circuit 140. In this embodiment, the controller 200 bypasses the power stage 100 by placing the first and second output control switches S1 a, S1 b in the respective second (nonconductive or open) states, as well as by placing the bypass switch S2 in the conductive state, and by placing the input switch S3 in the second (nonconductive) state.

The provision of the input switch S3 in this embodiment advantageously disconnects the power cell 100 and the output 104 from the AC input source, whether from the secondary 34 in FIG. 1 or from another associated AC input source. As seen in FIG. 4, moreover, the controller 200 in certain embodiments selectively places the first and second output control switches S1 a, S1 b in the second state before placing the input switch S3 in the second state, and may place the input switch S3 in the second state (e.g., at time T3 in FIG. 4) before placing the bypass switch S2 in the conductive state. In these embodiments, the timing between the signals 212 (e.g., T2-T1, T3-T1 and T2-T3) can be set according to any of the above described considerations and can be selectively adjusted by the controller 200 based on one or more power converter conditions. As discussed above, moreover, other switching sequences and/or relative timing may be implemented in other embodiments, wherein the broader aspects of the present disclosure are not limited by the illustrated examples.

By the above techniques and apparatus, a given cell 100 may be effectively bypassed to allow continued operation of the power conversion system 10 regardless of the failure state of the cell 100. For example, if one of the upper switches Q1 or Q3 (FIG. 2 or 4) should fail in a conductive state, the output control switches S1 will open, thereby disconnecting the output load 50 (FIG. 1) from the upper DC link node 131. Likewise, even if one of the lower switches Q2 or cute for fails in a conductive state, the output 104 is electrically isolated from the lower DC rail 132. Thus, the present disclosure advantageously presents a significant advance over conventional multilevel converter cell bypass techniques and protects the output load 50 regardless of the failure condition of a given power cell 100 while allowing the power cell 100 to be bypassed for continued operation of the converter 10.

The above examples are merely illustrative of several possible embodiments of various aspects of the present disclosure, wherein equivalent alterations and/or modifications will occur to others skilled in the art upon reading and understanding this specification and the annexed drawings. In particular regard to the various functions performed by the above described components (assemblies, devices, systems, circuits, and the like), the terms (including a reference to a “means”) used to describe such components are intended to correspond, unless otherwise indicated, to any component, such as hardware, processor-executed software, or combinations thereof, which performs the specified function of the described component (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the illustrated implementations of the disclosure. In addition, although a particular feature of the disclosure may have been disclosed with respect to only one of several implementations, such feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular application. Also, to the extent that the terms “including”, “includes”, “having”, “has”, “with”, or variants thereof are used in the detailed description and/or in the claims, such terms are intended to be inclusive in a manner similar to the term “comprising”. the following is claimed: 

The invention claimed is:
 1. A power conversion system, comprising: a plurality of power stages connected in series to form a multilevel inverter circuit (40) for connection to a load, the power stages individually comprising: a switching circuit including a plurality of switching devices coupled between a DC link circuit and an output, the switching circuit operative according to a plurality of switching control signals to provide an output voltage having an amplitude of one of at least two discrete levels at the output, a first output control switch coupled between a first node of the switching circuit and a first output terminal, and a second output control switch coupled between a second node of the switching circuit and a second output terminal, the first and second output control switches each being operative in a first state to allow current to flow between the switching circuit and the output and in a second state to prevent current from flowing between the switching circuit and the output, and a bypass switch coupled across the output of the switching circuit, the bypass switch operative in a nonconductive state and a conductive state to bypass the output of the switching circuit; and a controller is operative to selectively bypass at least one power stage by placing the first and second output control switches in the respective second states and by placing the bypass switch in the conductive state.
 2. The power conversion system of claim 1, wherein the controller is operative to selectively place the first and second output control switches in the second state before placing the bypass switch in the conductive state.
 3. The power conversion system of claim 2, further comprising an input switch coupled between an AC input and the switching circuit of the at least one power stage, the input switch being operative in a first state to allow current to flow between the AC input and the switching circuit and a second state to prevent current from flowing between the AC input and the switching circuit, wherein the controller is operative to bypass the at least one power stage by placing the first and second output control switches in the respective second states, by placing the bypass switch in the conductive state, and by placing the input switch in the second state.
 4. The power conversion system of claim 3, wherein the controller is operative to selectively place the first and second output control switches in the second state before placing the input switch in the second state.
 5. The power conversion system of claim 4, wherein the controller is operative to place the input switch in the second state before placing the bypass switch in the conductive state.
 6. The power conversion system of claim 5, wherein the switching circuit includes four switching devices connected in an H-bridge configuration between the DC link circuit and the output, and wherein the controller is operative to provide the switching control signals to the four switching devices of the switching circuit to provide the output voltage having an amplitude of one of at least two discrete levels at the output.
 7. The power conversion system of claim 2, wherein the switching circuit includes four switching devices connected in an H-bridge configuration between the DC link circuit and the output, and wherein the controller is operative to provide the switching control signals to the four switching devices of the switching circuit to provide the output voltage having an amplitude of one of at least two discrete levels at the output.
 8. The power conversion system of claim 1, further comprising an input switch coupled between an AC input and the switching circuit of the at least one power stage, the input switch being operative in a first state to allow current to flow between the AC input and the switching circuit and a second state to prevent current from flowing between the AC input and the switching circuit, wherein the controller is operative to bypass the at least one power stage by placing the first and second output control switches in the respective second states, by placing the bypass switch in the conductive state, and by placing the input switch in the second state.
 9. The power conversion system of claim 8, wherein the controller is operative to selectively place the first and second output control switches in the second state before placing the input switch in the second state.
 10. The power conversion system of claim 8, wherein the switching circuit includes four switching devices connected in an H-bridge configuration between the DC link circuit and the output, and wherein the controller is operative to provide the switching control signals to the four switching devices of the switching circuit to provide the output voltage having an amplitude of one of at least two discrete levels at the output.
 11. The power conversion system of claim 1, wherein the switching circuit includes four switching devices connected in an H-bridge configuration between the DC link circuit and the output, and wherein the controller is operative to provide the switching control signals to the four switching devices of the switching circuit to provide the output voltage having an amplitude of one of at least two discrete levels at the output.
 12. A power cell for use as a power stage in a multilevel inverter circuit (40), the power cell comprising: an AC input for receiving AC input power; a rectifier coupled with the AC input; a DC link circuit coupled with the rectifier and including at least one capacitance coupled between first and second DC link nodes; a switching circuit including a plurality of switching devices coupled between the DC link circuit and an output, the switching circuit operative according to a plurality of switching control signals to provide an output voltage having an amplitude of one of at least two discrete levels at the output; a first output control switch coupled between a first node of the switching circuit and a first output terminal, and a second output control switch coupled between a second node of the switching circuit and a second output terminal, the first and second output control switches each being operative according to at least one output switching control signal in a first state to allow current to flow between the switching circuit and the output and in a second state to prevent current from flowing between the switching circuit and the output, and a bypass switch coupled across the output of the switching circuit, the bypass switch operative according to a bypass switching control signal in a nonconductive state and in a conductive state to bypass the output of the switching circuit.
 13. The power cell of claim 12, further comprising an input switch coupled between the AC input and the switching circuit, the input switch being operative according to an input switching control signal in a first state to allow current to flow between the AC input and the switching circuit and a second state to prevent current from flowing between the AC input and the switching circuit.
 14. The power cell of claim 13, wherein the switching circuit includes four switching devices connected in an H-bridge configuration between the DC link circuit and the output.
 15. The power cell of claim 12, wherein the switching circuit includes four switching devices connected in an H-bridge configuration between the DC link circuit and the output. 